The present invention relates to methods and apparatuses for identifying cells in a cellular communication system.
In the forthcoming evolution of the mobile cellular standards like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high capacity high data rate system in existing radio spectrum, a new system has to be able to utilize a bandwidth of varying size. A proposal for such a new flexible cellular system, called Third Generation Long Term Evolution (3G LTE), can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as the multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported for the largest bandwidth. However, it is expected that 3G LTE will be used not only for high rate services, but also for low rate services like voice. Since 3G LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over IP (VoIP) will likely be the service that carries speech.
One important aspect of LTE is the mobility function, hence synchronization symbols and cell search procedures are of major importance in order for the User Equipment (UE) to detect and synchronize with other cells.
The currently proposed cell search scheme for LTE is as follows:
1. Using the primary synchronization channel (P-SCH), detect the 5 ms timing of a new cell.
2. Detect frame timing and Cell group using the secondary synchronization channel (S-SCH).
3. Use the reference symbols (also called CQI pilots) to detect the cell ID. The interested reader is referred to the document R1-062990, entitled “Outcome of cell search drafting session”, TSG-RAN WG1 #46bis, Oct. 9-13, 2006 for more information about this proposal.
The first two steps are well known in the art and are similar to the cell search scheme presently used in WCDMA systems. The third step is also used in WCDMA, where the pilot signal (CPICH) is scrambled with a pseudorandom noise sequence (pn-sequence) that determines the cell ID. By assuming the channel that affects the CPICH over a certain interval (one or two slots in WCDMA) is constant, one can detect the scrambling sequence easily.
The idea in LTE is also to scramble the reference symbols, both with a pn-sequence and also with an orthogonal code. However, unlike WCDMA, LTE does not have strong continuous pilot channels, but instead relies on fewer reference symbols (RS). These RS are placed in the first and third from last OFDM symbol in each subframe, and are placed on every sixth carrier, hence a distance of 90 kHz between the pilots. This is illustrated in FIG. 1, which depicts the proposed pilot (reference signal) pattern in the frequency (f) and time (t) dimension for the LTE system. In the figure, the first reference symbols are denoted “R1”; the second reference symbols are denoted “R2”; and data are denoted “D”.
A fundamental problem with using pilot symbols that are transmitted on different sub-carriers for scrambling code identification is that the phases for the different sub-carriers are typically affected in different and unknown ways from one another. This means that, unlike in WCDMA systems in which the channel is constant over the one or two slots and hence no phase equalization is needed to perform cell ID detection, in LTE systems coherent alignment of the pilots without equalization is not feasible, making the scrambling detection procedure much harder in an LTE system than in earlier known systems. Some examples of how delay-spread and sampling error affect the channel for different sub-carriers are described in U.S. Patent Application Publication No. US-2008-0101488-A1 to Wilhelmsson and Lindoff entitled “Robust and Low-Complexity Combined Signal Power Estimation”.
Consequently, there is a need for algorithms that allow for coherent alignment of pilots in OFDM systems like LTE therefore making it possible to use the pilots for cell ID detection.